1. Field of the Invention
The invention relates to tire innerliner compositions and vulcanized innerliners made therefrom used as air impermeable layers in the construction of pneumatic tires.
2. Description of Related Art
Butyl rubber, i.e., elastomeric copolymers of isobutylene with up to about 10 wt % of isoprene, possesses excellent resistance to air permeability and good aging properties which render it quite suitable for use as tire inner tubes or innerliner material for the production of tubeless pneumatic tires. The innerliner is composed of a relatively thin sheet of the elastomer formulated with compounding additives and a curing system, which is laminated to the inner surface of a tire carcass layer of an uncured tire as the tire is formed on a tire building drum. Final cure of the composite structure produces a tire having a cured innerliner adhered to the carcass which serves as a barrier to the passage of compressed air through the tire.
Halogenated butyl rubber normally containing from about 0.5 to 3 wt % halogen, e.g., bromine or chlorine, has proven to be a more effective innerliner material because the halogenated polymer exhibits improved adhesion to the tire carcass material and facilitates tire assembly. The halogenated material can be formulated with a curative system, e.g., zinc oxide/sulfur curing agents, which contribute to the development of interfacial crosslinking between the surface of the innerliner layer and the surface of the adjacent carcass layer which normally contains a more highly unsaturated rubber, thereby enhancing adhesion of the innerliner to the carcass. The use of halogenated butyl rubber for tire innerliners is disclosed in U.S. Pat. No. 2,943,664, as well as numerous other patents.
More recently, a new class of halogenated C.sub.4 -C.sub.7 isomonoolefin elastomers have been developed which demonstrate superior heat aging and flex properties as compared with halogenated butyl rubber. These polymers comprise random interpolymers of C.sub.4 -C.sub.7 isomonoolefin, such as isobutylene, with up to about 20 wt % of a para-alkylstyrene, such as para-methylstyrene, containing from about 0.1 to about 5 mole % of halomethylstyrene groups, e.g., bromomethylstyrene groups. These materials are more resistant to heat aging because they are free of olefinic unsaturation and yet they provide the good resistance to air permeability, flex resistance, tensile strength elongation and adhesion properties desired for tire innerliner applications. These polymers may also be cured by facile crosslinking reactions involving the benzylic halogen atom using zinc oxide/sulfur curing systems similar to those used to cure halogenated butyl rubber. These halogenated polymers are further described in U.S. Pat. No. 5,162,445, and compositions containing these polymers used for the fabrication of tire innerliners are disclosed in PCT published application WO-A-92 01575.
There has developed increased concern in recent years that the disposal of worn out tires by incineration may lead to the development of toxic, halogen-containing gas by-products as the result of thermal decomposition of the innerliner component of such tires. This concern, whether real or unfounded, has led tire manufacturers to seek tire construction components which are essentially free of halogen or have at least a reduced halogen content. In the case of tire innerliner materials, this requires the provision of elastomeric materials which, on the one hand, possess good air impermeability, heat flex, heat aging and tire carcass adhesion properties and which, on the other hand, are also essentially free of halogen or at least contain a significantly reduced halogen content.